Dissolved sulphur dioxide or sulphite is usually considered to be a reducing agent. Further, it is known that the oxidation of sulphite is accelerated through exposure to UV radiation (Matthews, J. H. et.al. J. Am. Chem. Soc. 1917,39, 635). Matthews teaches, however, that oxidation is retarded by the presence of trace amounts of various species. In addition, no change in the oxidation state of these species was observed.
Many drinking water supplies across the world are contaminated by trace contaminants including arsenic, iron and manganese. World Health Organisation standards require very low levels of contaminants (for example arsenic-a 10 ppb limit). The presence of manganese gives rise to “dirty water” problems and can result in soiling of clothes and staining of household fixtures when present in concentrations in excess of 20 ppb in drinking water.
Many waste waters and mineral processing liquors from industry also include arsenic, iron, manganese and cerium, and in the field of nuclear technology, uranium.
As part of the removal process, chemical oxidants such as chlorine, ozone and permanganate are often used. However, these oxidants can give rise to harmful byproducts such as chloroform, and the presence of residual permanganate can produce discoloured waters.